Nature and extent of the burden and risks associated with participation, benefit and group relatedness: For this imaging study patients have to make maximal 6 extra visits to the clinic for screening, to receive tracer injection and for maximal 3 PET/CT scans and the biopsy before start of standard treatment with pembrolizumab. The study ends after the last imaging or biopsy (whichever comes first) and will approximately take 7 days. Whenever possible, all procedures that are part of the study protocol will be planned during regular visits to the hospital as part of care as usual.